Semiconductor device

ABSTRACT

A semiconductor device includes: a compound semiconductor substrate; a buffer layer, a channel layer, and a Schottky junction forming layer sequentially disposed on the compound semiconductor substrate, the buffer layer, the channel layer, and the Schottky junction forming layer each being compound semiconductor materials; a source electrode and a drain electrode located on the Schottky junction forming layer; and a gate electrode disposed between the source and drain electrodes and forming a Schottky junction with the Schottky junction forming layer. The dopant impurity concentration in the channel layer is inversely proportional to the third power of depth into the channel layer from a top surface of the channel layer. The gate electrode has a gate length in a range from 0.2 μm to 0.6 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices having improveddistortion characteristics.

2. Background Art

The trend toward digitization of communications systems has created agreat need to improve the distortion characteristics of thesemiconductor devices in the output stage of communication devices, aswell as to reduce the size and increase the output power and efficiencyof these semiconductor devices.

When two signals of different frequencies f1 and f2 (fundamentalfrequencies) are input to a semiconductor device, second harmonicshaving frequencies of f1×2 and f2×2 are typically generated and mixedwith the fundamental frequencies, forming additional signals, ordistortion components, at frequencies of 2×f1−f2 and 2×f2−f1, which arevery close to the fundamental frequencies. This type of intermodulationdistortion is referred to as “third order intermodulation distortion,”or “IMD3,” and caused by nonlinear characteristics of the semiconductordevice. Such intermodulation distortion may cause noise between adjacentlines. To prevent this, a communications system employing a plurality ofcommunication lines (or frequency multiplex communication) requiressemiconductor devices having low distortion characteristics.

The relationship between the distortion characteristics of asemiconductor device and electrical parameters thereof may be analyzedusing Volterra series representation. (See, e.g., R. A. Minasian, IEEETrans. Microwave Theory Tech., vol. 28, No. 1, pp. 1-8, 1980.) AVolterra series expansion for determining the IMD3 indicates thatincreasing the transconductance gm of the semiconductor device orreducing its second derivative gm″ (with respect to the bias voltage)may be effective in improving the distortion characteristics of thesemiconductor device.

An effective way (theoretically proven) to reduce the second derivative(gm″) of the transconductance is to establish a doping profile in thechannel layer of the semiconductor device such that the dopant impurityconcentration varies inversely with the third power of depth, x,measured into the layer from the surface (that is, the dopant impurityconcentration is proportional to x⁻³). (See, e.g., R. A. Pucel,Electronics Lett., vol. 14, No. 6, pp. 204-206, 1978.) We fabricatedsuch a semiconductor device and investigated its distortioncharacteristics, as described below.

FIG. 9 is a cross-sectional view of a conventional semiconductor device.Referring to FIG. 9, the following layers are sequentially formed on topof one another on a semi-insulating GaAs substrate 11: an undopedAlGaAs/undoped GaAs superlattice buffer layer 12, an undoped GaAs bufferlayer 13, an n-type GaAs channel layer 101, an n-type AlGaAs Schottkyjunction forming layer 15, an n-type GaAs lower contact layer 16, ann-type AlGaAs etch stopper layer 17, and an n⁺-type GaAs upper contactlayer 18.

A source electrode 19 and a drain electrode 20 are formed on the n⁺-typeGaAs upper contact layer 18. A recess structure is formed through then-type AlGaAs etch stopper layer 17 and the n⁺-type GaAs upper contactlayer 18. A gate electrode 21 is disposed within the recess structurebetween the source electrode 19 and the drain electrode 20 and forms aSchottky junction with the n-type AlGaAs Schottky junction forming layer15.

The n-type GaAs channel layer 101 has a graded doping profile such thatthe dopant impurity concentration varies inversely with the third powerof depth, x, measured into the layer from the surface (that is, thedopant impurity concentration is proportional to x⁻³). The n-type GaAschannel layer 101 has a thickness of 1800 Å, and its top surface has adopant impurity concentration of 2.3×10¹⁷ cm⁻³.

FIG. 10 is a diagram showing measured distortion characteristics(namely, adjacent channel power, or ACP) of this conventionalsemiconductor device. FIG. 10 also shows, for comparison, measureddistortion characteristics (or adjacent channel power) of a widely usedsemiconductor device in which the channel layer has a uniform dopantimpurity concentration as a function of depth. The channel layer of thecomparative semiconductor device is made of n-type GaAs and has a dopantimpurity concentration of 1.5×10¹⁷ cm⁻³ and a thickness of 1300 Å. Thatis, this channel layer has substantially the same sheet dopant impurityconcentration and the same pinch-off voltage as the channel layer of thesemiconductor device described above and having the graded channeldoping profile.

These semiconductor devices have a gate length Lg of 1.1 μm and a gatewidth Wg of 12.6 mm. They were each mounted in a surface mount discretepackage. The distortion characteristics of each semiconductor devicewere measured when a 2.14 GHz W-CDMA modulated signal (3GPP TEST MODEL1, 64 code single signal, 3.84 MHz channel bandwidth) was input to thedevice with the drain voltage Vd and the drain current Id set to 10 Vand 300 mA.

The measurement results clearly show that the semiconductor devicehaving the graded channel doping profile has lower distortioncharacteristics than the comparative semiconductor device having theuniform channel doping profile.

FIG. 11 shows measured relationships between the drain current Id andthe transconductance gm of the semiconductor device having the gradedchannel doping profile and between the drain current Id and the secondderivative gm″ of the transconductance gm (with respect to the gatebias). FIG. 11 also shows the same relationships for the comparativesemiconductor device having the uniform channel doping profile.

As shown in FIG. 11, these semiconductor devices have substantially thesame transconductance (gm). However, the transconductance gm of thesemiconductor device having the graded channel doping profile has asmaller second derivative (with respect to the gate bias) than that ofthe comparative semiconductor device having the uniform channel dopingprofile. From this, it may be concluded that the improved distortioncharacteristics of the semiconductor device having the graded channeldoping profile results from the fact that its transconductance gm has areduced second derivative gm″ (with respect to the gate bias).

However, generally the characteristics of a field-effect semiconductordevice tend to be affected by its surface state, as well as by the stateof its buffer layer side. This means that it is difficult to achieveideal transconductance characteristics (that is, transconductancecharacteristics whose second derivative gm″ with respect to the gatebias is zero) by the creation of the above channel doping profile alone.

SUMMARY OF THE INVENTION

The present invention has been devised to solve the above problems. Itis, therefore, an object of the present invention to provide asemiconductor device having improved distortion characteristics.

According to one aspect of the present invention, a semiconductor devicecomprises: a compound semiconductor substrate; a buffer layer, a channellayer, and a Schottky junction forming layer sequentially formed on topof one another on said compound semiconductor substrate, said bufferlayer, said channel layer, and said Schottky junction forming layerbeing made of a compound semiconductor; a source electrode and a drainelectrode each formed on said Schottky junction forming layer; and agate electrode disposed between said source and drain electrodes andforming a Schottky junction with said Schottky junction forming layer;wherein the carrier density in said channel layer is inverselyproportional to the third power of depth into said channel layer from atop surface thereof; wherein said channel layer has a uniform sheetcarrier density; and wherein said top surface of said channel layer hasa doping density of 5.0×10¹⁷ cm⁻³-2.0×10¹⁸ cm⁻³.

Thus, the present invention provides a semiconductor device havingimproved distortion characteristics.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor device according toa first embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between depth into thechannel layer and the carrier density.

FIG. 3 is a diagram showing a measured relationship between thedistortion characteristics of the semiconductor device of the firstembodiment and the doping density of the top surface of its channellayer.

FIG. 4 is a diagram showing measured relationships between the draincurrent Id and the transconductance gm of a semiconductor device andbetween the drain current Id and the quantity |gm″/gm³| for differentdoping densities of the top surface of the channel layer.

FIG. 5 is a diagram showing a measured relationship between thedistortion characteristics of the semiconductor device of the secondembodiment and its gate length.

FIG. 6 shows measured relationships between the drain current Id of asemiconductor device and the second derivative gm″ of itstransconductance and between the drain current Id and the quantity|gm″/gm³| for different gate lengths Lg.

FIG. 7 is a cross-sectional view of a semiconductor device according toa third embodiment of the present invention.

FIG. 8 shows a measured relationship between the drain current Id ofeach semiconductor device and the quantity |gm″/gm³|.

FIG. 9 is a cross-sectional view of a conventional semiconductor device.

FIG. 10 is a diagram showing measured distortion characteristics of thisconventional semiconductor device.

FIG. 11 shows measured relationships between the drain current Id andthe transconductance gm of the semiconductor device having the gradedchannel doping profile and between the drain current Id and the secondderivative gm″ of the transconductance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a cross-sectional view of a semiconductor device according toa first embodiment of the present invention. Referring to FIG. 1, thefollowing layers are sequentially formed on top of one another on asemi-insulating GaAs substrate 11 (referred to as a “compoundsemiconductor substrate” in the appended claims): an undopedAlGaAs/undoped GaAs superlattice buffer layer 12; an undoped GaAs bufferlayer 13 (referred to simply as a “buffer layer” in the appendedclaims); an n-type GaAs channel layer 14 (referred to simply as a“channel layer” in the appended claims); an n-type AlGaAs Schottkyjunction forming layer 15 (referred to simply as a “Schottky junctionforming layer” in the appended claims); an n-type GaAs lower contactlayer 16, an n-type AlGaAs etch stopper layer 17; and an n⁺-type GaAsupper contact layer 18.

A source electrode 19 and a drain electrode 20 are formed on the n⁺-typeGaAs upper contact layer 18 (i.e., above the n-type AlGaAs Schottkyjunction forming layer 15). A recess structure is formed through then-type AlGaAs etch stopper layer 17 and the n⁺-type GaAs upper contactlayer 18. A gate electrode 21 is disposed within the recess structurebetween the source electrode 19 and the drain electrode 20 and forms aSchottky junction with the n-type AlGaAs Schottky junction forming layer15. The gate length Lg of the gate electrode 21 is 0.5 μm-1.1 μm.

FIG. 2 is a diagram showing the relationship between depth into thechannel layer and the dopant impurity concentration. As shown in FIG. 2,the n-type GaAs channel layer 14 has a graded doping profile such thatthe dopant impurity concentration varies inversely with the third powerof depth, x, into the channel layer from the surface thereof (that is,the dopant impurity concentration is proportional to x⁻³). Further, then-type GaAs channel layer 14 has a fixed sheet dopant impurityconcentration of 1.24±0.1×10¹² cm⁻² (measured by Hall measurementmeans).

According to the present embodiment, the top surface of the n-type GaAschannel layer 14 has a dopant impurity concentration of 5.0×10¹⁷cm⁻³-2.0×10¹⁸ cm⁻³. More specifically, when the dopant impurityconcentration at the top surface of the n-type GaAs channel layer 14 is5.0×10¹⁷ cm⁻³, the thickness of the channel layer 14 is 700 Å; when thedopant impurity concentration of the top surface is 2.0×10¹⁸ cm⁻³, thethickness of the channel layer 14 is 180 Å.

There will now be briefly described a method for manufacturing thesemiconductor device according to the present embodiment. First, thefollowing layers are sequentially formed on top of one another on thesemi-insulating GaAs substrate 11 by a crystal growth technique such asMBE or MOCVD: the undoped AlGaAs/undoped GaAs superlattice buffer layer12, the undoped GaAs buffer layer 13, the n-type GaAs channel layer 14,the n-type AlGaAs Schottky junction forming layer 15, the n-type GaAslower contact layer 16, the n-type AlGaAs etch stopper layer 17, and then⁺-type GaAs upper contact layer 18. It should be noted that the n-typeGaAs channel layer 14 is doped with Si to form the doping profiledescribed above. Further, the top surface of the n-type GaAs channellayer 14 has a dopant impurity concentration of 5.0×10¹⁷ cm⁻³-2.0×10¹⁸cm⁻³.

The recess structure, the source electrode 19, the drain electrode 20and the gate electrode 21 are then formed by suitable processes,completing the manufacture of the semiconductor device of the presentembodiment.

Characteristics of the semiconductor device of the present embodimentwill now be described. FIG. 3 is a diagram showing a measuredrelationship between the distortion characteristics (namely, theadjacent channel power, or ACP) of the semiconductor device of thepresent embodiment and the dopant impurity concentration of the topsurface of its channel layer. This semiconductor device has a gatelength Lg of 1.1 μm and a gate width Wg of 12.6 mm. It was mounted in asurface mount discrete package. The distortion characteristics of thesemiconductor device were measured when a 2.14 GHz W-CDMA modulatedsignal (3GPP TEST MODEL 1, 64 code single signal, 3.84 MHz channelbandwidth) was input to the device with the drain voltage Vd and thedrain current Id set to 10 V and 300 mA. The output power (Pout) of thesemiconductor device was 24 dBm.

The measurement results show that the distortion characteristics of thesemiconductor device improved as the dopant impurity concentration atthe top surface of the channel layer increased (and the thickness of thechannel layer was reduced accordingly), while maintaining the gradeddoping impurity profile described above, until the dopant impurityconcentration at the top surface of the channel layer reached 5.0×10¹⁷cm⁻³, after which the distortion characteristics substantially did notchange. That is, the semiconductor device has low distortioncharacteristics when the top surface of its channel layer has a dopantimpurity concentration equal to or higher than 5.0×10¹⁷ cm⁻³. The dopantimpurity concentration of the top surface of the channel layer may beset to 2.0×10¹⁸ cm⁻³ or less to increase the thickness of the channellayer so as to be able to reliably form the above dopant impurityconcentration profile in the channel layer while sufficiently activatingthe carriers.

FIG. 4 is a diagram showing measured relationships between the draincurrent Id and the transconductance gm of a semiconductor device andbetween the drain current Id and the quantity |gm″/gm³| (where gm″ isthe second derivative of the transconductance gm with respect to thebias voltage) for different doping densities of the top surface of thechannel layer. This semiconductor device has a gate length Lg of 1.1 μmand a gate width Wg of 100 μm. The reason for the results shown in FIG.3 will be described with reference to FIG. 4.

As shown in FIG. 4, the transconductance gm uniformly increased as thedoping density at the top surface of the channel layer increased (andthe thickness of the channel layer was reduced accordingly) whilemaintaining the graded dopant impurity concentration profile describedabove. On the other hand, since the graded dopant impurity concentrationprofile described above was adopted, gm″ did not increase as gm wasincreased. As a result, the quantity |gm″/gm³|uniformly decreased as thedopant impurity concentration in the channel layer increased (and thethickness of channel layer was reduced accordingly).

It should be noted that a Volterra series expansion for determining thethird order intermodulation distortion (IMD3) includes the term|gm″/gm³|. Therefore, it may be concluded that the reduction in thequantity |gm″/gm³| (shown in FIG. 4) resulted in the improved distortioncharacteristics of the semiconductor device (shown in FIG. 3) when thedopant impurity concentration at the top surface of the channel layerwas increased (and the thickness of the channel layer was reducedaccordingly) while maintaining the graded dopant impurity concentrationprofile described above.

It should be noted that the n-type GaAs channel layer 14 may have astepwise-graded doping profile, instead of the continuously gradeddoping profile described above, such that the dopant impurityconcentration varies substantially inversely with the third power ofdepth, x, into the channel layer from the top surface of the channellayer (that is, the dopant impurity concentration is substantiallyinversely proportional to x⁻³), with the same distortion reducingeffect.

Second Embodiment

A second embodiment of the present invention provides a semiconductordevice in which the channel layer has a graded doping profile such thatthe dopant impurity concentration varies inversely with the third powerof depth into the channel layer from the top surface of the channellayer, as in the semiconductor device of the first embodiment. The gateelectrode (21) of this semiconductor device has a gate length Lg of 0.2μm-0.6 μm. Unlike the first embodiment, the doping impurityconcentration at the top surface of the channel layer is not limited toany particular range. The following example assumes that the channellayer has a thickness of 700 Å and its top surface has a doping densityof 5.0×10¹⁷ cm⁻³. Except for the features described above, thesemiconductor device of the present embodiment is similar to that of thefirst embodiment. Further, it is manufactured in the same manner asdescribed in connection with the first embodiment.

Characteristics of the semiconductor device of the present embodimentwill now be described. FIG. 5 is a diagram showing a measuredrelationship between the distortion characteristics (namely, theadjacent channel power, or ACP) of the semiconductor device of thepresent embodiment and its gate length.

The gate width Wg of this semiconductor device was 12.6 mm. The gatelength Lg was changed from 0.5 μm to 1.1 μm. The device is mounted in asurface mount discrete package. The distortion characteristics of thesemiconductor device were measured when a 2.14 GHz W-CDMA modulatedsignal (3GPP TEST MODEL 1, 64 code single signal, 3.84 MHz channelbandwidth) was input to the device with the drain voltage Vd and thedrain current Id set to 10 V and 300 mA. The output power (Pout) of thesemiconductor device was 24 dBm.

The measurement results show that as the gate length Lg was reduced, thedistortion characteristics of the semiconductor device improved untilthe gate length Lg reached 0.6 μm, after which the distortioncharacteristics substantially did not change. That is, the semiconductordevice has low distortion characteristics when the gate electrode 21 hasa gate length Lg equal to or smaller than 0.6 μm. The gate electrode 21may have a gate length Lg equal to or larger than 0.2 μm, which allowsthe electrode to be formed by a common process.

FIG. 6 shows measured relationships between the drain current Id of asemiconductor device and the second derivative gm″ of itstransconductance gm (with respect to the gate bias) and between thedrain current Id and the quantity |gm″/gm³| for different gate lengthsLg. The gate width Wg of this semiconductor device was 300 μm. The gatelength Lg was changed from 0.5 μm to 1.1 μm. The reason for the resultsshown in FIG. 5 will be described with reference to FIG. 6.

As shown in FIG. 6, as the gate length Lg was reduced, the secondderivative gm″ of the transconductance of the semiconductor deviceuniformly decreased and hence the quantity |gm″/gm³| also uniformlydecreased.

As described above, a Volterra series expansion for determining the IMD3includes the term |gm″/gm³|. Therefore, it may be concluded that theabove reduction in the quantity |gm″/gm³| (shown in FIG. 6) resulted inthe improved distortion characteristics of the semiconductor device(shown in FIG. 5) when the gate length Lg was reduced.

The reason for the decrease in the second derivative gm″ when the gatelength Lg was reduced may be that the pinch-off voltage significantlydecreased due to the short-channel effect, which resulted in an changein the gm characteristics such that the second derivative gm″ wasreduced.

Third Embodiment

FIG. 7 is a cross-sectional view of a semiconductor device according toa third embodiment of the present invention. This semiconductor devicediffers from that shown in FIG. 1 in that it includes an undopedAl_(0.20)Ga_(0.80)As buffer layer 22 (referred to simply as a “bufferlayer” in the appended claims) instead of the undoped GaAs buffer layer13 (also referred to as a “buffer layer” in the appended claims). Itshould be noted that undoped Al_(0.20)Ga_(0.80)As, from which the bufferlayer 22 is formed, has a lower electron affinity than GaAs, from whichthe n-type GaAs channel layer 14 (referred to simply as a “channellayer” in the appended claims) is formed. Unlike the first embodiment,the doping density of the top surface of the channel layer is notlimited to any particular range. Except for the features describedabove, the semiconductor device of the present embodiment is similar tothat of the first embodiment. Further, the device is manufactured in thesame manner as described in connection with the first embodiment.

Characteristics of the semiconductor device of the present embodimentwill now be described. FIG. 8 shows characteristics of a semiconductordevice having an undoped Al_(0.20)Ga_(0.80)As buffer layer and thathaving an undoped GaAs buffer. Specifically, FIG. 8 shows a measuredrelationship between the drain current Id of each semiconductor deviceand the quantity |gm″/gm³| (where gm is the transconductance of thedevice and gm″ is the second derivative of the transconductance gm withrespect to the gate bias). These semiconductor devices have a gatelength Lg of 0.5 μm and a gate width Wg of 300 μm. The measurementresults clearly show that the semiconductor device with the undopedAl_(0.20)Ga_(0.80)As buffer layer has a lower value of the quantity|gm″/gm³| than the semiconductor device with the undoped GaAs bufferlayer when Id≦40 mA/mm. This results from the fact that thetransconductance (gm) of the semiconductor device with the undopedAl_(0.20)Ga_(0.80)As buffer layer has a smaller second derivative gm″(with respect to the gate bias) than the transconductance (gm) of thesemiconductor device with the undoped GaAs buffer layer when Id≦40mA/mm.

The reason for this fact is that since undoped Al_(0.2)Ga_(0.80), As hasa lower electron affinity than undoped GaAs, the semiconductor devicewith the undoped Al_(0.20)Ga_(0.80)As buffer layer 22 has a betterdopant impurity concentration profile in the n-type GaAs channel layer14 than the semiconductor device with the undoped GaAs buffer layer 13due to the increased electron confinement effect caused by the increasedhetero barrier (between the buffer and channel layers). (That is, thechannel layer of the semiconductor device with the undopedAl_(0.20)Ga_(0.80)As buffer layer has a doping profile such that thedopant impurity concentration is more nearly inversely proportional tothe third power of depth, x, into the channel layer from the surfacethereof.)

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2007-131467,filed on May 17, 2007 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

1. A semiconductor device comprising: a compound semiconductor substrate; a buffer layer, a channel layer, and a Schottky junction forming layer sequentially laminated on said compound semiconductor substrate, each of said buffer layer, said channel layer, and said Schottky junction forming layer being compound semiconductor materials; a source electrode and a drain electrode, each of said source electrode and the drain electrode being disposed on said Schottky junction forming layer; and a gate electrode disposed between said source and drain electrodes and forming a Schottky junction with said Schottky junction forming layer, wherein said channel layer has a dopant impurity concentration that varies with distance from a top surface of said channel layer and is inversely proportional to the third power of distance into said channel layer from said top surface of said channel layer, and said gate electrode has a gate length in a range from 0.2 μm to 0.6 μm. 